J. Am. Chem. SOC.1992,114, 9686-9688
9686 scheme 11'
& Hla ; 40% (Ring A: cis/trans=3/2) n=2 j HID ; 30% (Rlng A: cis/trans=3/2)
9
h-l
1p n.1
(16
n.2
&3 Hla
55% (Ring A: cls/lrans=l/l) aHHIDl a ;;34% (Ring A: cis/trans=l/i)
n=l
n=l 11.90%; (fling A: cls/trans=7/2) n=2
12
89%; (Ring A: cls/trans=7/2)
Bb H I P ,,=2
l.5~ H l a HIP
n.1
1z
n-2
l8
'Reagents: (a) PhH, Ph3P, 12, imidazole; (b) allyltributyltin,AIBN, PhH, 80 OC; (c) NaH, 3-MeOC6H4CH2Cl,Bu,NI, DMF; (d) I(~oll)~ClO~, CH2C12;(e) Zn, EtOH, reflux; (f) Ac20, DMAP, EtOAc; (8) (Me),C=CHCH,Br, NaH, Bu,NI, DMF; (h) 9-BBN, THF, then Na202;(i) Swern's oxidation; (j) Ph3P--CH2,THF; (k) MeOH, HCI. Scheme 111
21
and furanoside substrates. Since these test compounds are very sterically crowded and conformationally restricted, this reaction should be equally successful for simpler systems, where the transition state for this step is expected to be less highly strained. It should also be noted that products arising from direct electrophilic attack on the nucleophilic residues were not observed in any of these cases. Since these nucleophiles are very electron rich, electrophile-nucleophile compatibility problems are not anticipated for less electron rich nucleophiles. The relative reactivities observed for the different substrates are interesting from a mechanistic standpoint. Since the intramolecular trapping of 3 is expected to be fast, the higher reactivities for the more strained furanoside compared to the pyranoside substrates in both ROS (10,
